It is well known that the useful life of a catalyst is limited. In the case of a catalyst used in hydrocarbon conversion, for example, its active sites may be poisoned by contaminants in the feedstock or the sites may become blocked by the build-up of unwanted by-products of reaction. Further, a catalyst may be deactivated by incorrect storage conditions.
As examples of contaminants in hydrocarbon feedstocks there may be mentioned oxygen-, nitrogen-, and sulphur-containing compounds. It has been found that certain upstream processes in the petrochemical industry form nitriles that are deleterious to catalyst activity. It has recently been discovered that certain sulphur-containing compounds are deleterious, especially those with high desorption temperatures.
In molecular sieve-catalysed olefin oligomerization or aromatic compound alkylation processes, it has been found that carbonaceous deposits, typically of a higher molecular weight and often referred to as “coke”, block not only the active sites on the surface of the molecular sieve, but also the pores of the catalyst, preventing access of reactants to active internal sites as well.
A spent catalyst may be discarded, but disposal may be economically or environmentally unacceptable. A catalyst may be regenerated, by which term is meant the restoration of the activity of the catalyst to or very near to its original activity. Many regeneration methods, however, require high temperatures involving the removal of the catalyst from the reactor, often to a remote location, and lengthy reactor downtime and often substantial expense may be involved. An alternative is catalyst rejuvenation, by which term is meant increasing the activity of a deactivated (a term used to include partially deactivated) catalyst, but not necessarily to its original activity. Rejuvenation methods may be carried out more easily than regeneration, resulting in decreased reactor downtime, in some instances in situ.
U.S. Pat. No. 4,550,090 describes the regeneration of a deactivated dewaxing catalyst, in particular a ZSM-5 type catalyst, including the removal of nitrogenous contaminants, by treatment of the catalyst with a base, e.g., NH4OH, and solvent-extraction.
WO 01/80995 describes rejuvenating a crystalline molecular sieve, especially a SAPO or AlPO4 type catalyst, deactivated by moisture by treatment with an anhydrous liquid or vapour.
U.S. Pat. No. 5,059,738 describes the reactivation of a catalyst in a process converting methanol to gasoline between about 300° C. and 400° C. in contact with a stream of inert purge gas. The inert gas may include nitrogen, light paraffinic hydrocarbons, and Group VIII gases of the Periodic Table of the Elements. The methanol to hydrocarbon conversion processes, such as the methanol-to-olefins (MTO) process and the methanol-to-gasoline (MTG) process, are known to occur via alkylation and dealkylation reaction steps involving aromatic intermediates. The “coke” formed in these processes therefore contain significant amounts of single up to 4 or 5 multiring aromatics. When the process uses a large pore open structure molecular sieve as catalyst, such as ZSM-5, single ring aromatics are sufficiently small to escape from the catalyst and appear in the product.
U.S. Pat. No. 4,417,086 describes a fluidized bed oligomerization process wherein periodically the flow of feed into the reaction zone may be stopped and the product may continue to be stripped from the catalyst with a stripping gas, which may be nitrogen. The oligomerization feed needs to contain gaseous olefins, and the oligomerization is operated with the olefin feedstock in the gas phase. The activity in such a gas phase oligomerization is significantly lower than with the oligomerization processes where the olefin feedstock is either partially or entirely in the liquid phase, or in the supercritical condition. The gas phase process therefore typically operates at a higher temperature as compared with these other processes, typically above 300° C., where side reactions become significant, such as cracking, olefin disproportionation, hydrogen transfer and dehydrocyclization. These side reactions cause the formation of byproducts such as paraffins, polyunsaturates, aromatics and olefins of other carbon numbers than the true oligomers of the feedstock olefins. These byproducts are acceptable, or even desirable, in certain product uses such as in transportation fuels, but they represent an undesired selectivity loss, and often an unacceptable product contamination, when the oligomer products are intended for the production of chemical derivatives such as alkylates or oxo-alcohols for plasticizers or detergents. In the gas phase oligomerization process of U.S. Pat. No. 4,417,086, the oligomers formed do not readily come off the catalyst, and they therefore are particularly prone to participate in these side reactions. Some of the byproducts, such as the aromatics, are intermediates for the formation of a particular kind of “coke”, containing single and multiring aromatics. That aromatic-containing “coke” is hard to remove from molecular sieve catalysts, and when such deactivated catalysts are rejuvenated, temperatures of above 300° C. are required. There is even no evidence in U.S. Pat. No. 4,417,086 that the rejuvenation at 316° C. is effective in removing also the polynuclear aromatic coke present on the catalyst or trapped in the catalyst pores. Since the typical operating conditions of the gas phase oligomerization process are in the same range, also above 300° C., the equipment complies with the design requirement suitable for this temperature range and the necessary auxiliary equipment is in place and adequate to reach those temperatures. The rejuvenation with inert gas above 300° C. therefore does not create an additional burden or complexity on a gas phase oligomerization process.
Oligomerization processes using molecular sieve catalysts at conditions wherein the feedstock is partially or entirely in the liquid phase or in the supercritical or dense phase condition typically operate at temperatures of 300° C. and below. This suppresses side reactions such that higher selectivities to desired true oligomers can be achieved, and the products are of high purity, suitable for the production of chemical derivatives such as alkylates or oxo-alcohols for plasticizers or detergents. Equally important, the carbonaceous deposits formed under these conditions have been found to be predominantly non-aromatic, and to have a hydrogen to carbon atom ratio of between 1.6 and 2.0. If the higher temperature rejuvenation process known from the gas phase oligomerization process, i.e. above 300° C., are to be applied, additional requirements are put on the equipment designs and on the auxiliary equipment that are not needed for the oligomerization process itself.
The same applies even more to processes for the alkylation of an aromatic compound with an olefinic alkylating agent. The operating temperatures of these processes are typically similar to those of liquid or dense phase oligomerization when the olefinic alkylating agent is ethylene, and even lower when the olefinic alkylating agent is propylene or a normal butene such as butene-1 or butene-2. Alkylation of an aromatic compound with an olefinic alkylating agent is carried out in both liquid and vapour phase reactor systems. The rejuvenation method of the invention is believed to be more suitable for liquid phase operation because of the lower process temperatures.
There therefore remains a need for a rejuvenation method, applicable to molecular sieve catalysts aged, i.e. deactivated, by use in an olefin oligomerization process under conditions whereby the feedstock is in the liquid phase or in the supercritical condition, or aged, i.e. deactivated, by use in a process for the alkylation of an aromatic compound with an olefinic alkylating agent, that does not bring with it the additional requirements on the equipment designs nor the need for auxiliary equipment that is not needed for the oligomerization or alkylation process itself.
We have now found that the high molecular weight carbonaceous deposits in the oligomerization processes wherein the feedstock is in the liquid phase or in the super-critical condition, or in a process for the alkylation of an aromatic compound with an olefinic alkylating agent, is different and of a softer, non-aromatic nature, and that the molecular sieve catalysts deactivated by use in such processes can be rejuvenated at milder conditions at or below 300° C. This means that the need for more stringent equipment design criteria and for extra auxiliary equipment can be avoided.
There remains a need for a method that rejuvenates a molecular sieve catalyst that has been deactivated by, for example, a feedstock contaminated with sulphur and/or nitrogen compounds.
EP-A-716 887 describes reactivating a solid acid catalyst, in particular a solid phosphoric acid catalyst, in situ by subjecting it to sub-atmospheric pressure, removal of material released by this means optionally being assisted by introducing an inert gas, e.g., nitrogen, into the reactor while evacuating.
In a number of other prior proposals, e.g., EP-A-1 070 694 and U.S. Pat. No. 4,560,536, nitrogen is used to purge a catalyst bed before regeneration or rejuvenation, for example by burning with oxygen or solvent-extraction.
However, these references neither disclose nor suggest that the contact with nitrogen would itself, i.e., without the necessity of any other treatment, effect rejuvenation. Indeed, from the conditions of contact with nitrogen disclosed (for example, the low partial pressure inherent in the procedure mentioned in EP-A-716 887 and the scavenging conditions of the other two references) when compared with the intensity of the treatments that follow, it seems unlikely that any significant reaction was to take place between the nitrogen and the catalyst as such.